The invention relates in general to a passive transponder used for the localization of people and of objects with the help of a radio transmitter which transmits RF-energy on one frequency and with the help of a radio receiver which receives RF-energy retransmitted on another frequency by the transponder.
U.S. Pat. No. 4,331,957 describes a passive transponder used for the rescuing of skiers who have been caught in avalanches. The transponder is glued onto a ski boot. The transponder includes an antenna in the shape of a metal foil with two main surfaces and a diode connected between the main surfaces. A mobile radio transmitter with a thereto-connected directional antenna emits radio frequency energy on a base frequency of 915 MHz. A mobile radio receiver, built together with the radio transmitter, is tuned to double the base frequency, 1830 MHz, and is connected to the directional antenna. The signal from the transmitter is modulated with an audio frequency within the audible range. If the transponder is touched by the transmitted signals the diode generates overtones of the base frequency. The first harmonic (double the base frequency) has high energy and is detected by the radio receiver. The rescue people hear this as a tone and can, through taking a bearing with the help of the directional antenna, determine the position of the victim of the avalanche. The big advantage of this searching method is the short time that it takes to investigate the avalanche area.
U.S. Pat. No. 4,656,478 discloses a transponder similar to the one above. The transponder comprises a dielectric support, an antenna and a covering layer. The antenna has a cut out portion, the edge of which defines a conductive line which is closed by a passive component so as to form a self-induction loop. The self-induction loop together with the capacitance of the passive component provide a circuit resonating at the frequency at which the transponder receives its energy. The transformation by the antenna of the energy received by the transponder at the base frequency f0 into energy available for retransmission by the transponder at a harmonic of frequency f0 is achieved with a better yield since the couple self-induction-internal capacitance of the passive component brings about an increase in the voltage at which the transformation is produced. The increase corresponds to the quality factor of the resonating circuit.
U.S. Pat. No. 4,890,111 discloses a transponder similar to the one mentioned in said latter US patent. The antenna elements of the transponder are formed by a metallic ribbon arranged in a planar loop surrounding the cut-out portion. The result of this arrangement is that for equal dimensions the capacitance of the stray capacitor formed by the antenna elements and the body of the person bearing the transponder is much less than in the transponder of the prior art. The arrangement will reduce the influence said stray capacitor will have on the resonating frequency. A T-shaped slot provided in the antenna elements provides a production advantage in that the gain of the transponder is much more constant from one transponder to another than in the case the antenna has no T-formed slot.
U.S. Pat. No. 5,223,851 relates to a miniature transponder including a magnetic antenna with a coil connected to an integrated circuit. In response to a signal received by the antenna the integrated circuit generates an identifying signal which is returned to the antenna for retransmission. A tube of a heat shrinkable material surrounds the transponder and protects it from mechanical shocks. This solution is fundamentally different from the two mentioned above for two reasons: It is based on a single frequency system as opposed to a harmonic (doubled) frequency system, and utilises low frequency as opposed to microwave frequencies.
The human body acts as a water surface that reflects received RF-energy. It is desirable that the RF-waves transmitted by the transponder on the double base frequency and the RF-waves reflected by the human body on the double base frequency are substantially in phase with each other so that the two reflected RF-waves constructively amplify each other. In this way, the RF-power of the received RF-waves on the double base frequency will be maximal. In order to achieve this the transponder should be placed at a certain given distance from the human body. With the given base frequency, this distance is long. So long that in practice it is inappropriate to have an air space between the transponder and the human body. According to the U.S. Pat. No. 3,331,957 the transponder is glued on the outside of a ski boot made of plastic which from a technical point of view means that a dielectric made of plastic is placed between the transponder and the foot, and thereby said given distance is reduced to a practically usable distance.
The applicant has found that a problem occurs if the transponder is mounted in a ski boot made of plastic. The RF-power emitted from the transponder on the double base frequency is reduced. The applicant found that the search equipment must be tuned to a lower frequency compared to when the transponder was glued on the outside of the ski boot in order for the RF-power emitted from the transponder on the double base frequency to be able to be detected with the maximal signal strength. Detection with the maximal signal strength is namely critical in the case that the transponder is at a large distance from the antenna, in which case the signal strength at the receiver is low. It namely must never be so low that the detection of the transponder is completely excluded.
It is desirable that the same search equipment shall be able to be used for the detection of transponders which are glued on boots, respectively for the detection of transponders which are built into boots. Returning of the search equipment is not possible in practice.
A drawback with the transponders of the first two US patents mentioned is that they are sensitive to the environment of the antenna. In particular their respective impedances are influenced by the surroundings of the antenna. A varying antenna impedance results in a degraded RF power retransmitted by the transponder at the first harmonic of the basic frequency.
One object of the invention is to provide a transponder that provides an optimum yield of the RF energy received on the base frequency and the RF energy retransmitted on a first harmonic of the base frequency.
Another object of the present invention is therefore to provide a transponder the impedance of which is generally independent of the surroundings of the antenna.
The invention has the object of avoiding the above-mentioned inconvenience with built-in transponders. This is achieved with the help of the features stated in claim 1.
The advantage that is achieved with the invention is that the near field of the antenna is substantially not, or only to a small degree, influenced by the surroundings of the antenna.
Another advantage which is achieved with the invention is that the dielectric which surrounds the transponder concentrates the RF-energy to a transmission line whereby the influence of the surroundings on the transponder""s characteristics are reduced.
In this document the expression dielectric means a material of which the dielectric constant is greater than 1. Through changing the transmission line geometry and the dielectric characteristics of the immediate surroundings of the transmission line an optimal relationship can be obtained between the electrical parameters for the frequencies f and 2f. In this way it is possible to manufacture a transponders which are matched to each given positioning of the transponder, for example in or on a ski boot, a jacket, a lifejacket or the like.
None of the US patents above discloses a matching network for matching the impedance of the passive component to the impedance of the antenna. In particular none of the US patents disclose an impedance matching transmission line.
Furthermore, none of the US patents above discloses a dielectric material surrounding the transmission line so as to concentrate the energy transported by the transmission line to the transmission line itself, thereby making the transmission line generally independent of the surroundings of the transponder.
Finally, none of the US patents discussed above discloses a dielectric material surrounding the antenna so as to reduce the influence the antenna""s surroundings on the near-field of the antenna.